Vitreous

Vitreous

Gel-like structure

Fills the vitreous chamber

Spherical except for Patellar Fossa

80% of entire globe

Vitreous: Function

1. Physical Support

   • Holds retina in place next to choroid

     • Neural retina + choroid only connected at optic disc and ora serrata

   • Shock absorber

     • Protects retina during eye movements, physical activity

2. Transmit + Refract Light

   • Minimal light scatter due to:

     • High [water]

     • Hyaluronic acid/collagen complex spacing

3. Diffusion Barrier

   • Between anterior and posterior segments

   • Prevents topical medications from reaching the retina/optic nerve

   • Entrance of medications from bloodstream rarely make it to anterior segment

4. Metabolite Storage/Buffer

   • Reservoir for metabolites of:

     • Retina

     • Lens

     • Ciliary Body

Vitreal Attachments

Strongest → Weakest

1. Vitreous Base

   • Strongest attachment (2)

   • At ora serrata (1)

     • 1.5-2 mm anterior

     • 1-3 mm posterior

   • Vitreal fibers embedded in BM of nonpigmented epithelium of the CB + ILM of retina

2. Posterior to lens

   • Hyaloideocapsular ligament (of Weiger)

   • Retrolental ligament

     • Forms retrolental space of Berger

   • Annular attachment

   • 1-2 mm wide; 8-9 mm diameter

   • Firm attachment in youth, diminishes after age 35

3. Optic Disc (Peripapillary)

   • Also diminishes with age

4. Macular

   • 3-4 mm in diameter

5. Retinal Vessels

   • Consists of fine strands that extend through ILM

   • May account for hemorrhages that occur with vitreoretinal traction

Vitreous Zones

1. Vitreous Cortex

   • AKA: Hyaloid Surface

   • Outer zone

   • Tightly packed collagen fibrils

     • Parallel + perpendicular to retinal surface

   • Anterior Vitreous Cortex:

     • Anterior to vitreous base

     • Adjacent to CB, posterior chamber, + lens

   • Posterior Vitreous Cortex

     • Posterior to vitreous base

     • In contact with the retina

     • Contains transvitreal channels that appear as holes

   • Posterior Vitreous Cortex: Transvitreal Channels

     • Prepapillary hole

     • Premacular hole

     • Prevascular fissures

2. Intermediate Zone

   • Continuous + unbranched fine fibers

   • Run anteroposteriorly

   • Arise near vitreous base, insert into posterior cortex

   • Vitreous tracts

     • Areas having different fiber densities

3. Cloquet’s Canal

   • AKA hyaloid channel, retrolental tract

   • Center of vitreous body

   • Former site of hyaloid artery system

   • Arises in retrolental space

   • Terminates in area of Martegiani

     • Funnel-shaped space at optic nerve

Vitreal Composition

98.5-99.7% water

Dilute solution of salts, soluble proteins, + hyaluronic acid

Collagen meshwork (insoluble protein)

Three major components (besides water):

1. Collagen

   • Collagen content highest:

     • Vitreous base > posterior cortex > anterior cortex > center

   • Mostly Collagen Type II

   • 3 identical α-chains form a triple helix

   • Collagen fibrils interconnect with hyaluronic acid

2. Hyaluronic Acid

   • Glycosaminoglycan

   • Long, unbranched coiled molecule

   • Hydrophilic

   • Maintains wide spacing between fibrils

   • Stabilizes the network formed by collagen strand

   • Concentration distribution similar to collagen

3. Hyalocytes

   • Vitreal cells (specific to vitreous)

   • Single, widely spaced layer in cortex near vitreal surface

   • Synthesize hyaluronic acid & Glycoproteins

   • Phagocytic properties- break down metabolic waste

   • Fibroblasts also located in vitreous base,

     • May be mistaken for hyalocytes

     • Synthesize collagen fibrils

     • Active in pathology

Vitreal Composition: Change with Age

Liquefaction

Infant vitreous: homogenous gel

With age, gel volume decreases & liquid volume increases

• 40 yrs: 80% gel/20% liquid

• 80 yrs: 50% liquid

Exact molecular mechanism unclear

Hyaluronic acid is redistributed from the gel to the liquid

Most liquefaction in center, where there is the least amount of collagen

Gel dissolves and is replaced with lacunae

Lacunae melt together over time

Age-related vitreal changes are associated with age related ocular disease:

• Nuclear sclerotic cataract

• Diabetic retinopathy

Intact, normally composed vitreous provides “protection”

• Brings nutrients to ocular structures

• Takes waste, metabolites, bad stuff away

Clinical Connection: Floaters

As vitreous gel liquefies, collagen complexes form clumps & bundles

Shadows cast on retina, seen as floating spots

Most noticeable on white wall/blue sky

May be a sign of retinal pathology

Clinical Connection: Posterior Vitreous Detachment (PVD)

Collapse of the vitreous body

• Cortex sinks to center

Due to normal liquefaction and age changes

Posterior vitreous detaches from the retinal internal limiting membrane at the peripapillary ring

May occur prematurely in cases of trauma, high myopia, etc.

Weiss Ring (Senile Annular Ring)

• Glial tissue from peripapillary attachment may remain in vitreous after PVD

• Appears as a circular ring on examination, just over the nerve

• Patient may see large floaters for days/weeks after detachment

During acute PVD stage, patients may notice an increase in floaters

If there is a particularly strong attachment between the posterior hyaloid and the ILM of the retina, a PVD may result in:

• A retinal break (tear, hole)

• Vitreo-retinal traction, especially in the foveal region

Monitor patients 4-6 weeks after PVD onset for retinal complications

Clinical Connection: Vitreoretinal Traction

The Vitreoretinal Interface

Vitreous cortex attached to ILM at :

• Vitreous base

• Optic disc

• Macula

• Vessels

Strongest in youth

Traction may occur with

• Liquefaction changes

• Trauma

Clinical Connection: Vitreoretinal Traction + Floaters

Treatment

• Monitor

• Vitrectomy

• Medication/Injectables

• Laser surgery- target disruptive floaters but not recommended